Micro-expression refers to brief, involuntary facial expression shown by a person that indicate emotions currently being experienced by such person. They may indicate subconscious responses by a person (thus indicating some reaction that the person does not even know that he or she is having) and can indicate the person's inner emotions even when the person is actively trying to hide emotions. Micro-expressions are very brief, typically on the order of 1/25 to 1/15 of a second. Micro-expressions may be interpreted in certain situations so as to classify the emotions of a person as one or more of emotional labels: disgust, anger, fear, sadness, happiness, contempt, surprise, amusement, embarrassment, anxiety, guilt, pride, relief, contentment, pleasure, and shame.
Micro-expressions have been sub-classified into three main groups according to how an expression is modified. First, “simulated expressions” are micro-expressions that are not accompanied by a genuine emotion (a brief flash of expression that then returns to a neutral state), which is the most commonly-studied form of micro-expression because of its nature. Second, “neutralized expressions” occur when a person suppresses a genuine expression and the face remains neutral, and is generally not observable because of its suppression. Third, “masked expressions” occur when a real expression is completely masked by a false expression.